TEHRAN, Iran — Iran's government threatened harsh action Saturday if opposition supporters take to the streets again to demand a new election in open defiance of the country's supreme leader.

Web sites run by supporters of opposition leader Mir Hossein Mousavi called for street protests at 4 p.m., but the candidate himself issued no public statement.

Fire trucks took up positions in Revolution Square, the site of the planned gathering, and riot police surrounded Tehran University, the site of recent clashes between protesters and security forces, one witness said.

Mousavi did not attend a meeting of the country's top election authority.

The decision to rally or stay off the streets will be crucial for the opposition movement and the unprecedented threat it poses to Iran's cleric-led government. Rallying could spark a bloody crackdown on Mousavi's supporters, or greatly weaken the government by forcing it to back away from its threat of violence. If the opposition heeds the official warnings and cancels, it could give the government the opening it needs to suppress its greatest internal challenge since the 1979 Islamic Revoultion.

It was not clear if Mousavi has the power to halt street demonstrations even if he orders his supporters to stay inside. The government appeared to be preparing for a confrontation.

The government statements were the most explicit warnings yet of force against protesters who gathered in massive rallies last week to demand the government cancel and rerun elections that ended with a declaration of overwhelming victory for hardline President Mahmoud Ahmadinejad. Mousavi says he won but Ahmadinejad stole the election through widespread fraud.

Supreme Leader Ayatollah Ali Khamenei sided firmly with Ahmadinejad Friday, saying the result reflected popular will.